The present invention relates to a method and apparatus for monitoring flow conditions within a borehole or the like filled with a substantially incompressible fluid and more particularly to such a method and apparatus wherein the test interval is defined by a packer assembly preferably comprising means forming at least one additional guard zone in order to facilitate measurement of flow conditions in the borehole.
In situ measurement of permeability in various underground formations has long been of importance for example in connection with oil and gas wells, including tight gas sand formations, also in connection with development of underground storage facilities for waste nuclear materials and in mining operations, particularly those employing solution mining or leaching techniques.
In applications of the type described above, it is essential to accurately characterize permeability of the underground formations of interest. In many of these applications, the underground formations have minimum permeability resulting in limited flow between the formation and the test interval so that it is particularly difficult to obtain accurate information. For example, in waste nuclear storage facilities, minimum permeability is essential in order to assure that the nuclear materials will not leach or seep into the underground formation and escape from the storage facility over long periods of time.
In the prior art, substantial effort has been expended in developing techniques for characterizing permeability of such underground formations by studying fluid flow characteristics for the formation. The relationship between permeability and flow characteristics has been well established, for example, under Darcy's Law or modifications thereof which define the relationship of permeability in connection with fluid flow through a substrate in response to a given pressure differential or head. Additional parameters such as porosity, saturation, fluid viscosity, threshold pressures, temperature, previous testing history, fracture extent, etc. may be of importance in accurately determining such permeability values.
A guarded straddle packer assembly is disclosed in a co-pending application entitled "Method and Apparatus for In Situ Determination of Permeability and Porosity" filed by Peter L. Lagus and Edward W. Peterson and assigned to the assignee of the present invention, now U.S. Pat. No. 4,353,249 issued October 12, 1982. That reference sets forth further information and additional prior art references concerning the measurement or inference of permeability from flow characteristics within a test interval defined along a borehole or the like. As is further described in that reference, such permeability tests may be conducted by measuring flow characteristics in either "in-flow" or "out-flow" tests depending upon the relative pressures in the test interval and in the surrounding formation. In any event, a pressure differential is established therebetween for purposes of inducing flow as a means for determining or inferring permeability and other formation characteristics.
The method and apparatus of the above noted reference is adapted for facilitating flow measurements in formations characterized by much lower permeability than has generally been measurable in the prior art. The guarded straddle packer assembly of the above noted reference was described in connection with fluid systems, either gases or liquids, within its test interval. However, it was found to be substantially more difficult to conduct such tests where the fluid filling the test interval consisted of a substantially incompressible liquid. In such situations, volumetric flow between the test interval and the surrounding formation tends to be even less than in similar situations where the test interval and surrounding porous formation are filled with a gas or the like.
The substantially greater mass of a substantially incompressible liquid creates additional effects tending to interfere with accurate measurements of flow characteristics and associated determination or inference of permeability values. Initially, within a gaseous system, it is relatively easy to conduct the fluid to the surface for measurement of certain characteristics. However, with a liquid being contained in the borehole test interval located for example thousands of feet below the surface, the need for such surface measurements creates a very substantial hydrostatic head interfering with or even preventing accurate measurements of the type contemplated by the present invention.
Use of a substantially incompressible liquid component within the test interval also tends to create or amplify "compliance effects" which may similarly interfere with or prevent accurate measurement of flow characteristics. In this regard, the term "compliance effects" refers to volumetric changes which may take place in any part of the system, either within the liquid itself, the packer system or even the borehole walls, particularly as a result of substantial pressure variations occurring within the test interval.
Accordingly, there has been found to remain a need for a method and apparatus for more accurately monitoring conditions within a borehole test interval containing an essentially incompressible liquid.